The invention relates to a device suitable for scanning a first and a second type of optical record carrier in a first and a second mode, respectively, each type of record carrier having an information plane formed by a structure arranged in tracks and containing information. The first type of carrier is designed to be scanned by radiation of a first wavelength and the second type is a phase structure designed to be scanned by radiation of a second wavelength, the first wavelength being shorter than the second wavelength. Such a device comprises a radiation source for supplying a radiation beam of the first wavelength, a lens system for focusing the radiation beam to a scanning spot on the information plane, a first detection system for converting scanning spot radiation from the information plane into an information signal, and a tracking servosystem controlled by a tracking error signal for keeping the scanning spot on the centerline of a track to be scanned in the first mode.
The need for such a device has arisen due to the development of novel radiation sources which produce radiation at a shorter wavelength than has hitherto been common practice, such as short-wavelength laser diodes and radiation sources constituted by the combination of a conventional diode laser and a frequency doubler. If such a novel radiation source is used in a read device, this device may form a smaller read spot. As a result, it will be possible to read novel record carriers with smaller information details and a larger information density than usual. Due to the large numbers of conventional record carriers already in use, it is very desirable that the novel read device is also suitable for reading these record carriers.
A device of the type described in the opening paragraph is known from the abstract of the Japanese Patent Application no. 2-83830. This device can read record carriers with different information densities by adapting the dimension of the read spot on the information plane to the information density, i.e. the read spot is enlarged for reading a record carrier having larger information details. In the known device the spot is enlarged by moving a plane-parallel plate into the converging radiation beam. As a result, the focus of the beam shifts along the optical axis and enlarges the cross-section of the beam at the area of the information plane. A drawback of this method is that there is generally little space between the lens system and the record carrier so that the interpositioning of an additional plate involves a greater risk of the record carrier touching the plate and being damaged due to shocks against the device. Another drawback is that when changing the type of record carrier, a component, the plate in this case, must be displaced, which is quite cumbersome at a location where there is little space. Further, a phase structure in an information plane designed for reading at a long wavelength is not always found to be satisfactorily readable with a beam having a short wavelength.